The present invention is directed in general to a neutron detection device and a method of manufacturing a neutron detection device. The present invention is specifically directed to a semiconductor device for the detection of neutrons that utilizes a neutron conversion layer in close proximity to a conventional memory cell structure.
The development of nuclear weapons gave rise to several urgent applications for highly sensitive neutron detectors. The applications included safeguarding nuclear materials and weapons, treaty verification, anti-proliferation, and the recovery of lost military payloads. More recently, however, the need to guard against nuclear smuggling, the potential of a radiological weapon (so called “dirty” bombs), and terrorist acts, has given rise to an urgent need to perform neutron surveillance at border and port facilities, transportation systems and other places where large amounts of a cargo or people pass by or through on a regular basis. Such neutron surveillance must be accomplished without undue restriction or disruption of traffic flow and events.
One class of conventional neutron detectors has been based on the phenomenon of scintillation, which is a result of photon-emitting transitions that occur in the wake of energetic charged nuclei released from reactions between incident neutrons and atomic nuclei. Scintillation devices include a light-transmissive neutron sensitive material (either a gas or a liquid) that generates light upon receipt of incident neutrons. The scintillation devices are typically coupled to a photomultiplier tube to generate an analog electrical signal based on the production of the light within the scintillation material. The analog signal is a representation of the incident neutron irradiation. Another class of conventional neutron detector is the gas filled counter, typically based on gaseous helium-3 contained in high pressure tubes. In particular, the helium-3 filled tubes are delicate, require careful handling, and can indicate false positives when abruptly moved or struck. These types of conventional neutron detectors are effective in many types of field operations, but they are not suitable for operations requiring compact and highly sensitive devices capable of functioning for long periods of time with low power consumption.
With the advent of solid state electronics, it was realized silicon-based semiconductor devices could be used to sense alpha particles emitted from a neutron converter material in which an (n. alpha) reaction had taken place. The role of the converter material is to convert incident neutrons into emitted charged particles which are more readily sensed. When the emitted charged particle transits a semiconductor device, it liberates charges in its wake, and these charges may be collected and used to sense the event stimulated by the initial neutron reaction. Such devices therefore serve as neutron detectors. Initial demonstrations of such a concept used free standing converter foils placed near a silicon detector such as a PIN diode. It is more common now to utilize films of converter material placed in contact with or deposited directly upon semiconductor detectors. Lithium metal has been used for this purpose, although the chemical reactivity of the lithium metal leads to shorter detector life. Greater life has been obtained with compounds of lithium such as LiF. a hard crystalline material. Boron metal has also been applied directly to silicon devices. See, “Recent Results From Thin-Film-Coated Semiconductor Neutron Detectors”, D. S. McGregor et al., X-Ray and Gamma-Ray Detectors and Applications IV, Proceedings of SPIE, Vol. 4784 (2002), the contents of which are incorporated herein by reference.
The use of diode structures in neutron detectors, however, has its own set of drawbacks and limitations. The internal noise level of an uncooled diode is appreciable, and consequently it is difficult, if not impossible, to measure low background levels of ambient thermal neutrons in the surrounding area or to detect single neutron events. A typical diode also has a thick semiconductor layer in which charges are collected. Charges liberated by gamma rays are also collected in the thick semiconductor layer and these charges contribute to the non-neutron noise signal of the detector.
More recently, it has been proposed that a previously considered disadvantage of semiconductor memory cells be turned into an advantage with respect to neutron detection. Memory cells can be “hardened” against radiation to prevent errors induced by radiation. In fact, the importance of such memory integrity has been readily appreciated for many years in the field of computers, aviation and space flight. A radiation-induced bit error is known as a soft error if the affected memory cell subsequently responds to write commands. In contrast, the induced bit error is known as a hard error if subsequent attempts to change the state of the memory cell are ineffective. Both hard and soft errors are known as single event upsets (SEUs) or single event errors (SEEs) provided that a single incoming particle induces the error in the memory cell. The error events, which are detrimental when trying to maintain data integrity, can be used in a positive manner to detect radiation events by simply monitoring the radiation-induced charges in the states of the memory cells.
Attempts have been made to utilize commercial memory circuits with a neutron converter in order to use the SEU associated with the memory circuits for neutron detection. For example, boron has been used in the semiconductor industry as a dopant and in boron containing glass as a passivation layer that is used to cover the circuit-defining structures and to encapsulate a finished semiconductor chip. It has been demonstrated that 10B in the dopant or borophophosilicate glass (BPSG) passivation layer is responsible for sensitizing a circuit to neutron radiation. See, “Experimental Investigation of Thermal Neutron-Induced Single Event Upset in Static Random Access Memories”. Y. Arita et al., Jpn. J. Appl. Phys. 40 (2001) pp L151-153, the contents of which are incorporated herein by reference. Accordingly, proposals have been made to coat boron on a conventional semiconductor memory chip containing a passivation layer or to first remove the passivation layer and then coat the chip with a boron converter material. U.S. Pat. No. 6,075,261 issued to Houssain et al. and entitled “Neutron Detecting Semiconductor Device”, the contents of which are incorporated herein by reference, discloses one such attempt at utilizing a conventional semiconductor memory structure as a neutron detector, wherein a neutron-reactant material (converter) is coated over a conventional flash memory device. Alpha particles emitted by the boron typically must pass through the structural layers which define the circuit before they reach the active semiconductor. These efforts to date, however, have resulted in insensitive detectors primarily because the boron conversion material is not located close enough to the active semiconductor layer. Thus, alpha particles generated by the boron conversion material dissipate their energy in the intervening material and cannot generate a sufficient charge in the active semiconductor layer to cause an SEU.
In view of the above, it would be desirable to provide a neutron detection device that does not require the use of high pressure tubes or high voltages, is not sensitive to gamma radiations, is not sensitive to thermal noise, and operates with low power consumption, but yet is sensitive enough to permit the counting of single neutron events.
It would further be desirable to provide a neutron detection device of inexpensive design and manufacture.
Still further, it would be desirable to provide a method of manufacturing a neutron detection device that involved the modification on conventional memory devices, thereby permitting conventional memory devices to be converted to neutron detection devices.